In critically ill and unstable patients assessment of oxygen delivery to the tissues is of vital importance. If inadequate, early interventions to optimise oxygen delivery may prevent multiple organ failure and death1. These interventions include administration of intravenous fluids, inotropes (that stimulate heart contraction) and support of ventilation to improve oxygenation of blood.
Central venous or mixed venous blood oxygen saturations reflect the adequacy of oxygen delivery to the parts of the body from which the blood has drained. Mixed venous blood (blood in the right ventricle and central and peripheral parts of the pulmonary arteries) offers the best assessment of the adequacy of oxygen delivery to the whole body. However, central venous blood (blood in the internal jugular, subclavian, femoral and brachiocephalic veins, the inferior and superior vena cava and the right atrium) can be used as a surrogate of the adequacy of oxygen delivery to the whole body.2 
Currently, assessment of oxygen delivery by venous saturation measurement is generally undertaken by placing a catheter in a central vein or pulmonary artery from which blood is withdrawn. Oxygen saturation of the withdrawn blood is then measured by a blood gas machine. Alternatively, a fibre-optic catheter can be placed in the central vein or pulmonary artery and the oxygen saturation can then be directly measured by optical methods. An approach such as this involving the insertion of an intravenous fibre-optic catheter and direct measurement of oxygen saturation by oximetry is discussed in U.S. Pat. No. 5,673,694 to Rivers.
Both of these approaches involve significant limitations as they require a skilled doctor to insert the catheter, they involve the expense of the blood gas machine or fibre-optic catheter, there is significant risk of adverse events associated with catheter insertion (pneumothorax, infection, bleeding, arrhythmia and tamponade) and finally, there is a delay in obtaining the venous blood saturation while the catheter is inserted.
The present inventor proposes a non-invasive method to directly measure blood oxygen saturation (such as central venous and mixed venous blood oxygen saturation) by placing a light oximeter device on the skin over deep vascular structures. Pulse oximetry, using red and infrared light sources, is an established technique to measure haemoglobin oxygen saturation of blood vessels in the skin. The sensors are commonly placed on fingers, ears, nose and forehead. Pulse oximetry is routinely used in patients to determine whether oxygenation of the blood by the lungs is adequate. Standard pulse oximetry techniques do not provide information about adequacy of oxygen delivery.
Two wavelengths of light are generally used in pulse oximetry one in the red band (between about 620 nm and about 750 nm, but usually in the range of about 640 nm-680 nm, most usually about 660 nm) and the infrared band (between about 750 nm and about 1 mm, but usually between about 900 nm and 960 nm, but often 905 nm, 910 nm or 940 nm). The light is absorbed by haemoglobin in the blood. Deoxyhaemoglobin (Hb) absorbs more of the red band while oxyhaemoglobin absorbs more of the infra-red band. In pulse oximetry light is first transmitted through the tissues and the intensity of the transmitted or reflected light is then measured by the photo-detector. The pulse oximiter determines the AC (pulsatile) component of the absorbance at each wavelength and the amount of the red and infrared AC components is determined, which is indicative of the concentration of oxyhaemoglobin and deoxyhaemoglobin molecules in the blood. The ratio of these molecules indicates the overall haemoglobin oxygen saturation.
The potential of non-invasive trans-cutaneous pulse oximetry to measure the haemoglobin oxygen saturation of blood in deep vascular structures, for example that carry central venous and mixed venous blood, has not previously been recognised. However, a recent patent (U.S. Pat. No. 7,047,055, to Boas and Zourabian3) has suggested that light oximetry of deep tissue structures is possible. This work demonstrated a light oximetric technique to measure arterial saturation in the head of a fetus in utero.
Other techniques have been proposed to measure mixed venous oxygen saturation using pulse oximetry. These techniques are, however, invasive and require insertion of an endotracheal tube (U.S. Pat. No. 6,961,600, Kohl)4 or a transoesophageal echocardiographic probe5. Venous saturation of peripheral tissues may also be measured using oximetric techniques. These measurements are, however, of limited clinical utility as they only reflect the extent of oxygen delivery to the peripheral tissue assessed, such as the index finger (US 2005/0256386, Chan) or thenar eminence (U.S. Pat. No. 7,072,701, Chen) (U.S. Pat. No. 6,985,763 Boas).
US patent publication no. 2006/0253007 to Cheng et al describes a light oximetric technique to measure cardiac output, by determining venous blood oxygen saturation in a few deep vascular structures. Cheng et al suggests the concurrent use of ultrasound to assist the correct location of emitter and receiver probes, as well as requiring oximetry measurements be taken simultaneously at two separate locations to distinguish the signal arising from the deep vascular structure from that of surrounding tissue. The present inventor has demonstrated that by utilising the pulsatile nature of the deep vascular structures to generate a plethysmographic trace it is possible to accurately locate the emitter and receiver elements to optimise the signal detected and to thereby do away with the need for concurrent ultrasonography and measurements from more than one location. The individuality of the plethysmography in the present technique is used to identify that the signal is arising from the vascular structure of interest and to filter out signals arising from other interfering chromophores, such as small blood vessels and surrounding tissues.
It is a preferred object of the present invention to overcome or at least ameliorate to some extent problems associated with prior art methods of determining oxygen saturation in deep vascular structures. Other objects of the present invention will become apparent from the following detailed description thereof.